BACKGROUND: High frequency of co-occurring substance use disorders and psychiatric disorders is reported in several studies. An important issue is whether we can rely on patient reports only. The purpose of this investigation was to find the prevalence of substance use among younger psychotic inpatients in Norway and to compare patients' self reports with the results of screening for drugs. MATERIAL AND METHODS: 65 patients aged 17 to 40, consecutively admitted to Blakstad Psychiatric Hospital in 2001 with psychosis were interviewed with regard to substance use 30 days prior to admittance. The Addiction Severity Index (EuropASI) was used for interviews. Blood and urine samples for drug analysis were taken at admittance. RESULTS: 54% of the patients reported having used one or more substances for intoxication during the month prior to admittance. 40% of the included patients had used illegal drugs. Laboratory analyses revealed use of illegal drugs, mostly cannabis and amphetamine, in 34% of the patients. Only one patient tested positively for a drug not reported in the interview. INTERPRETATION: The investigation shows that 54% of the younger psychotic inpatients intoxicate themselves with legal or illegal substances. This fact is important for the total medical management of these patients in psychiatric hospitals. We also found that this group of patients were reliable in reporting their substance use when a standardised interview was used.